Metallic cases have the potential to offer designers the freedom to make mobile devices very thin. There is design trend toward all-metal cases, but there is also a fundamental limitation to the percentage of the mobile device case area that can be metallic.
Slots in the surface of the metallic case may be used to obtain acceptable radiation performance. However, when the size of the mobile device is small compared to the frequency of operation, the inefficient radiation and narrow-band nature of slot antennas are drawbacks. Furthermore, slots are highly susceptible to detuning by the presence of the user's relatively high dielectric and lossy tissue. To combat its narrow band nature, a slot antenna can be made tunable to cover an instantaneous bandwidth. However, due to the wide bandwidth used by the Long Term Evolution (LTE)-advanced protocol, tuning of single slot antennas cannot cover all instantaneous bandwidths required for future wireless platforms.